1. Field of the Invention
The present invention relates to transparent metal stacks.
2. Related Art
Micro-electro-mechanical-switches (MEMS) have been used in such applications as pressure sensors, accelerometers, and nozzles, and have been proposed for use in radio frequency (RF) telecommunications systems. In particular, a number of different types of MEMS switches have been developed. Petersen, K. xe2x80x9cMicromechanical Membrane Switches on Silicon,xe2x80x9d IBM J. Res. Develop., vol. 23, 1979, pp. 376-385 describes a chemical etching process for fabricating a mechanical switch, which is sensitive to vibrations and has poor insertion loss and isolation. Gretillat et al, xe2x80x9cElectrostatic Polysilicon Microrelays Integrated with MOSFETs,xe2x80x9d in proceedings of Micro Electro Mechanical Systems Workshop, 1994, pp. 97-101 describes a switch for use in an automated testing applications. The switch exhibits large insertion loss and high frequency capacitive coupling to its polysilicon cantilever arm in its off-state. Yao et al. xe2x80x9cA Surface Micromachined Minature Switch for Telecommunications Applications with Signal Frequencies from DC up to 4 GHzxe2x80x9d In Tech. Digest, Transducer-95, Stockholm, Sweden, Jun. 25-29, 1995, pp. 384-387 describes a switch for use in RF telecommunications that uses electrostatic actuation to control a silicon dioxide cantilever arm to open and close a signal line, and has an electrical isolation of xe2x88x9250 dB and an insertion loss of 0.1 dB at 4 GHz. These three documents are incorporated in their entireties herein by reference.
The fields of communications and data processing are currently transitioning from using electrical signals to using optical signals. As a result, there is an increased need for optical devices that perform various tasks in the control of these optical signals. Such devices include tunable filters and optical limiters.
One method of creating a low distortion, controllable photonic delay is through the use of photonic band gap (PBG) structures. Uniform PBG structures typically comprise a stack of alternating layers of refractive materials of similar thicknesses, such as gallium arsenide and aluminum arsenide, which exhibit photonic band gaps in their transmission spectra. These alternating layers have different indices of refraction and can be deposited by well known deposition techniques onto a substrate.
By sending a photonic signal of a given frequency (xcfx89) through a uniform PGB device, the discontinuity of the indices of refraction imparts a delay to the photonic signal. These devices slow down the photonic signal as a result of scattering inside the uniform PBG structure. Since the photonic delay is proportional to the square of the number of periods contained in the uniform PBG structure, a device can be constructed that imparts a predetermined delay to a photonic signal. The physical processes involved in the photonic signal delay imparted by a uniform PBG structure are described in detail in Scalora, et al., xe2x80x9cUltrashort pulse propagation at the photonic band edge: large tunable group delay with minimal distortion and loss,xe2x80x9d Phys. Rev. E Rapid Comm. 54(2), R1078-R1081 (August 1996), which is incorporated by reference herein in its entirety.
With the above methodology, an external electric field is applied in order to shift the location of the transmission resonance inside a photonic band gap device to induce changes in the velocity of an externally injected pulse of light. By varying the strength of the applied field, a method by which the index of refraction of the affected material layer can be changed. Changing the refractive index of the layer causes the desired change in the velocity of the incident light beam.
However, the index of refraction of most ordinary materials can be changed only slightly with the utilization of externally applied electric fields. For example, the index of refraction of GaAs can be changed by approximately one part in 1000 if an ordinary electric field is applied across the 100-nm layer discussed above. That is, a shift in the index of refraction occurs from 3.4 to 3.401. While this shift can be considered meaningful, experimentally observable, and useful for some applications like an optical delay line, this shift is too small and impractical for many other applications of interest. As an example, this change in index of refraction from 3.4 to 3.401 can shift the transmission resonance in a photonic band gap structure by approximately 0.5 nm. While this shift may be adequate for control of the velocity of an optical pulse, it is completely inadequate for device applications such as optical limiters and tunable filters wherein device requirements can be very demanding. For example, an optical limiter must stop a coherent signal regardless of its wavelength. This means it must distinguish between low intensity light levels, such as those of ambient light, and a high intensity coherent light, such as a laser beam. In addition, the device must be able to discriminate between different colors of the incident light, coherent or not, over the entire visible range. That is, it must have a dynamic range approximately 1000 times greater than the shift discussed in our previous patent application and incorporated by reference herein in its entirety, i.e., from 0.5 nm to approximately 500 nm or more.
Hence, there is a need for a device and method to change the index of refraction by greater than a factor of 2 in a number of readily available materials.
The present invention generally relates to a device and method of optics propagation and signal control integrated with micro-electro-mechanical-switches (MEMS). In particular, the present invention relates to modifying optical transmission properties of a transparent, multilayer metal stack by mechanically varying the thickness of an air gap between layers in the stack. This is accomplished with the novel approach of utilizing MEMS coupled with the stack to change the index of refraction in a given layer of the transparent multilayer metal stack.
According to one embodiment of the present invention, this is accomplished by developing a hybrid combination of transparent multilayer stacks and MEMS, wherein an air gap is used as one of the dielectric layers. The air gap thickness can be controlled by the MEMS device thereby enabling much greater control of the index of refraction.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.